RIFTING INITIATION THROUGH LATERAL VARIATIONS OF LITHOSPHERIC BASAL STRESS BENEATH PREEXISTING ZONES OF WEAKNESS

Classic rifting initiation and early-stage development models include passive rifting in which stretching is attributed to far-field horizontal extension probably originated from subduction zones, and active rifting which advocates the role of a local mantle plume. Recent seismological investigations especially those conducted in the incipient southern and southwestern sections of the East African Rift System suggest that neither models can satisfactorily explain rifting initiation. Instead, those studies, together with the observation that the vast majority of the world’s Cenozoic rifts were developed in ancient orogenic belts sandwiched between ancient continental blocks which are zones of mechanical weakness and sudden changes in lithospheric thickness, strongly suggest that rifting is initiated by lateral variations in horizontal stress applied to the base of the lithosphere by the relative movement between the lithosphere and the asthenosphere. Such differential stress can be created regardless the driving mechanism of plate motion, either by plate boundary processes such as ridge push or slab pull, or by actively convecting asthenospheric flow, as long as there is a significant change in lithospheric thickness and the existence of a mechanically weak zone associated with ancient orogenic belts. Results from the recent NSF-funded SAFARI (Seismic Arrays for African Rift Initiation) project will be presented to provide evidence for the rifting initiation model.